I received a phone call the other day that caught me a bit off guard. A customer was asking what tolerance they need to have on their motor shaft for it to fit with one of our incremental encoders. I reflexively told the customer that we machine our shaft I.D. to a –0.0000” +.0005” tolerance. He told me he already knew that, but what did we recommend for his shaft, the motor shaft that the incremental encoder was to be mounted on?

Since I am an Electrical Engineer, I wanted to make sure I had all of my ducks in a row before I took my second shot at answering a mechanical related question. I let him know that I would look up the information he needed and call him back. After I got off the phone I immediately knew what I should have told him. This answer may surprise you, but we don’t specify tolerances for the fit of our customer’s motor shafts. – not in the way one would expect. Instead of a fit tolerance, we have TIR and Endplay tolerances on total encoder movement after it is mounted.

The reason for this is because unlike modular incremental encoders, which rely on the mounting shaft to hold the disk and sensor air gap, the QD145 series of incremental encoders has an internal bearing set that maintains the air gap. This takes the need for an exacting precision shaft to shaft fit out of the list of problems motor manufacturers face when designing a new motor.

I called the customer back and let him know the good news; that instead of some tight machining numbers, he only needed to keep his QD145 Incremental encoder within .007” of radial shaft runout and within +/- .030” for axial shaft runout.